Pilot warning indicator system



Aug. 31, 1954 M. v. KlEBERT, JR

PILOT WARNING INDICATOR SYSTEM 2 Sheets-Sheet l Filed May 5l, 1945 tofs.

Aug. 31, 1954 Filed May 31, 1945 Planek'z l r4- d= Hor/Zonta! DistanceM. v. KIEBERT, JR 2,688,131

PILOT WARNING INDICATOR SYSTEM 2 Sheets-Sheet 2 s= Horizontal SpeedS=$lant Speed D-S/ant Distance Fig. 2

Targa] Dropping Distance in Ft.

Speed in Feet per Second Down Sweep v fr f jffm See.+

S Caunteron Lmec) *One Moda/at, Cycle -v Apparent lief/eetebl Wave Dueto Doppler E ffeot Reflected Wave at fixed Distance 'Transmitted WaveCounter oft Phase of Switching Voltage for Up Sweep Counter InventorCounter on lvlart/n I./ K/ebert dr.

Counter off Phase of Switching Voltage Down Sweep Counter Attorney forPatented Aug. 31, 1954 UNITED STATES PATENT OFFICE 7 Claims. (Cl. 343-7)(Granted under Title 35, U. S. Code (1952),

sec. 266) This invention relates in general to automatic load releaseequipment of the electronic computer type for use on aircraft such as,for eX- ample, in bombing surface targets.

The general object of the invention is to provide the pilot of theaircraft with a warning signal in advance of the time that the bomb orother load is automatically released from the plane. l

A more speciiic object is to provide the pilot with awarning signalwhich is derived from the same output which serves to effect release ofthe bomb or other load carried by the aircraft at the proper time asdetermined by the altitude of the attacking plane, its distance from thetarget and therelative speed at which the plane is coming on the target.When the pilot receives the warning signal, it thus indicates to himthat the electronic load equipment is functioning properly in computingthe time of load release and assures him that his bomb or other loadwill release automatically and at the proper time to strike the target.

A more specific object is to provide a warning signal of the typedescribed above from control apparatus comprising a pair of controltubes. The output circuit from the first tube is used to control therelease of the bomb or other load from the plane, and the output circuitfrom the second tube is used to actuate the alarm device previouslyreferred to. The two tubes are differently biased beyond cut-off. Thecontrol voltage appearing at the output of the computing device isapplied equally and simultaneously to both tubes. The second tube isbiased beyond cut-off to a lesser extent than is the first tube, andhence will reach conduction ahead of the rst tube. Thus, the alarmdevice will be actuated in advance of the time at which the load releasedevice is actuated.

The advantage of my warning signal is obvious, for should the pilot failto receive the Warning signal, he will then know that the computingequipment is not functioning properly and, hence, would not operate torelease the load automatically. Under such conditions, he must then acthimself to estimate the proper time at which the load should be releasedand effect such release through use of a manually operated load releasemechanism.

Referring now to the drawings, Fig. 1 illustrates a preferred embodimentof the invention as applied to a particular device for automaticallyeffecting the release of a bomb or other load from an aircraft l flyingat va comparatively low 2 altitude (5G-300 feet) and directed at anisolated or semi-isolated target such as a marine surface vessel Il;

Fig. 2 is a diagram illustrating the factors entering into the problemof computing the time of load release from the aircraft;

Fig. 3 is a plot of curves showing the relation between speed anddropping distance for several l different altitudes; and

Fig. 4 is a plot showing the relation between the transmitted andreflected frequency modulated Waves.

The bomb is caused to be released while the aircraft is approachingvessel Il at the proper time so that the bomb will strike it.Alternatively, by controls to be explained hereinafter, bomb release maybe effected at such time as will cause the bomb to strike the vessel atany preselected distance from 0 to 100 feet ahead of the target. Thislatter arrangement is of particular advantage when a skip bombingtechnique is employed.

The problem to be solved by the apparatus requires the followingconditions to be determined and evaluated:

(1) Height of the plane (or bomb) above sea level.

(2) Horizontal distance to the target (Range).

(3) Relative horizontal speed between plane and target (Range rate).

Plane I0 may be accurately flown at a preselected altitude with the aidof an altimeter of the FM radio type such as described in U. S. Patent2,206,903, issued July 9, 1940, to R. F. Lane et al. This will take careof condition 1. The horizontal distance to the target vessel, condition2, is indirectly determined by measuring the slant distance from planeIl) to target ll by the frequency modulation technique. Yagi typeantenna I2 is used to beam the high frequency radio waves forward, butsufficiently downward to be reected back from vessel Il to receivingantenna I3 also of the Yagi type. Thus the time required for the wave totravel from the plane I0 to target Il and back again, and hence thedistance, may be evaluated. Condition 3 is evaluated in terms of slantrelative speed between plane I0 and target I l by utilizing the Dopplereffect produced by the rapid approach of the plane l0 towards thetarget.

Employing well known mathematical relationships, the following equationrelating to Fig. 2 can be developed:

where DzProper dropping distance in feet measured on the direct slantpath.

SzRelative speed in feet per second, measured on the direct slant path.

AzAltitude in feet.

TzTotal time delay (inherent delay in response of system components):.36second.

4 ";11=Time of free space fall in seconds Equation 1 can then be used toplot a series of speed vs. distance curves for altitudes of 50, '75,100, 150, 200 and 300 feet, as shown by dotted lines in Fig. 3. Thesecurves do not pass through zero since the apparatus measures the slantspeed and slant distance, and when the airplane I is directly abovetarget I I, the distance to the target is the altitude of the airplane.These curves representing lEquation 1 are not used to calibrate theequipment because of the difficulty of compensating the equipment for anon-linear relationship between speed and distance. Rather,straight-line approximations to the curves are shown, and these areshown by the solid lines in Fig. 3.

The equations of the straight lines are of the mathematical form:

D=MS+du (2) the new factors being M :Slope of the straight line, anddri-:Distance intercept at Zero speed, in feet.

The distance intercept do corresponds in a general way to planealtitude, but because of the apfproximations made (i. e., total timedelay and linear approximation), departs considerably in numerical valuefrom the `planes actual altitude.

The present apparatus solves Equation 2 automatically for each ofseveral altitudes, the pilot operating a switch for this purpose. Themanner in which the solution is obtained is explained in the paragraphswhich follow. Since each altitude requires a different slope M, theapparatus is designed to give this required slope by varying thebandwidth of the frequency swept by the transmitter component of theapparatus. Hence, the bandwidth is a function of the time of drop plusthe delay due to mechanical and electrical inertia of apparatuscomponents.

In Fig. 1, a linear frequency modulated signal is generated in atransmitter oscillator I4. This signal is delivered to the transmittingantenna I2 and also is fed directly to a balanced heterodyne detectorI5. Linear frequency modulation of transmitter oscillator I4 is producedby a vibrating condenser type of frequency modulator unit IS. Modulatorunit I6 is comprised of a permanent magnetic field (not shown) and amoving coil Il driving a metal diaphragm I8 which acts as the movingplate of a frequency modulator condenser. A generator I9 driven by adynamotor 22 delivers a square wave voltage to a Wave-shaping circuit23, which converts the square wave to a substantially triangular one.The output from Wave Shaper 23 is applied to the moving coil II. Thisproduces a linear type of motion to diaphragm I8 which moves relative toa pair of xed condenser plates 2li and 25. The amplitude of the squarewave output from generator I9 determines the maximum f2 and minimum f1limits of modulation of the R. F. carrier signal generated in theoscillator I4, and such amplitude is adjusted by means of an alti- 4tude compensation switch 2G which is interposed between the triang-ularwave shaper 23 and modulator unit I6. rIIIhe period of the output fromsquare wave generator I9 is determined by the speed of dynamotor 22 andXes the repetition rate or modulation cycle of the modulation of thehigh frequency carrier wave generated in the transmitter oscillator I4.

Measurement of distance (distance voltage) The frequency modulated waveis transmitted from the plane It to and reflected back from the target ii. Referring to Fig. 4, the instantaneous frequency of the reflectedwave lags the instantaneous frequency of the transmitted wave by where Dis the distance from the transmitter to the target in feet, and C is thespeed of `light in feet per second. Since a portion of the transmitted'wave is fed direct from the transmitter M into detector I5, thetransmitted and reflected waves will heterodyne in detector I 5 toproduce a comparatively low frequency beat-note, the frequency of whichis proportional to target distance. In Fig. Vli, ,f1 and f2 define thelimits of R. F. sweep of the transmitter. For each complete modulationcycle, the total number of cycles per second swept through will then be2(12-11). If fm is Vthe modulation frequency in cycles per second, thenthe total number of cycles per second swept in one second will be 2fmf2fi The frequency of the signal reflected from the target I I is shownas the dotted line displaced from that for the transmitted signal by atime hence, the resultant instantaneous frequency difference fr betweentransmitted and reflected signals will be Where W=(f2-f1) 106 megacyclesper second per sweep. The number of cycles per second of beatnote perfoot of distance from plane I0 to target I I will then be Wfm 24:6

The foregoing thus gives information for the measurement of distancebetween plane and target in terms of beat-note output of the detectorI5. Furthermore, the number of cycles per second per foot of targetdistance can be varied directly by varying the sweep width W, which isdone conveniently by changingI the amplitude of the driving voltagesupplied by wave shaper v23 to the frequency modulator unit I6associated with the R. F. oscillator component I4 of the transmittingapparatus.

The sweep width W is set to give the proper slope to the calibrationcurve for each altitude by means of the altitude compensation switch 26which varies the voltage supplied to coil I'I of the condenser modulatorunit. The same switch also provides the proper amount of bias torepresent the distance intercept du.

Measurement of speed S (speed voltage) Since the distance from target Ilat which the bomb must be released from plane l is dependent upon theforward speed of the airplane relative to the target as well as thealtitude, it is necessary that this speed be evaluated by the apparatus.As previously explained, use is accordingly made of the so-calledDoppler effect produced by the approach of the plane to the target.

It can be shown that for the present apparatus,

where fd is the apparent received frequency, fu is the transmittedfrequency, C is the speed of the wave energy in feet per second, and Sis slant speed of airplane I0.

From Equation 5, the apparent increase in frequency due to the Dopplereffect is thus Frequency fo is the center or average F-M frequency andas such is a constant. Furthermore, the speed C of the radio wave isalso constant. Thus, the horizontal component of the Doppler frequency,i. e., the apparent change in frequency due to the Doppler effect, is aconstant times the relative slant speed of the plane I0 toward target Ildivided by cos 0, where `19 is as defined in Fig. 2.

Information in reflected signal Section A of Fig. 4 represents thetransmitted signal, the reected signal, and the apparent signal due toDoppler effect. As indicated, the apparent reflected signal will alwaysbe greater in frequency at any instant than the reflected signal withoutDoppler effect by an additional number of cycles equal to This is aconstant times the slant speed S of the plane. Evaluating the constant(Where for-410x106' cycles per second) results in the equation Dopplerfrequency fD=2`S= .8338' (6) Referring again to Fig. 4, the apparentreflected R. F. signal with Doppler will be at a time angle or delay ofseconds with respect to the transmitted signal. The resultantinstantaneous frequency difference with Doppler, between the transmittedand 6 reflected signals during the up-sweep is given by the equationTherefore, when the transmitted and reected signals are mixed in thedetector l5, two audio frequencies will occur,`one fu during the upsweepand the other fd during the down-sweep. For any given altitude, theequations may be written Where K and K1 are constants, D and S are theslant distance and relative slant speed, respectively, fr isthe beatfrequency resulting from distance, and fn is the Doppler frequencyresulting from relative speed.

Equations 10 and l1 indicate that the two frequencies will each becomposed of both distance and speed factors. The up-sweep frequency ,tuis the difference, and the down-sweep frequency fd is the sum, of thedistance and speed variables. The apparatus constantly measures thesetwo variables; when they represent a solution to Equation 2 in terms ofpre-setr altitude, a relay 21 is tripped and the bomb released fromplane I0. Actually, the apparatus subtracts Equation l1 from Equation 10(the K in Equation 11 is made different from the K in Equation 10), andWhen the di'erence is a solution to Equation 2, the bomb release relay21 is tripped.

Method of obtaining voltages proportional to distance and speed The lowfrequency beat signals from the detector I5 are put through amulti-stage amplifier unit 28 and then converted into square waves by alimiter stage 3| for delivery to a pair of differentially connectedcounters or frequency determining circuits 32, 33. The output of limiterstage 3l is fed through capacitor 34 into the up-sweep counter 33, andthrough capacitor 35 into the down-sweep counter 32. The up-sweep anddown-sweep counter circuits are keyed on and off by application of thesquare wave output from generator I9. As this is the same square waveoutput which furnishes the sweep voltage for the transmitter I4, keyingof the counters 32, 33 will always remain in a fixed phase relationshipwith the transmitter R. F. sweep. The square wave from generator I9 isfed without phase reversal to the down-sweep counter 32 but goes througha phase reversal of in a phase inverter 36 before keying the up-sweepcounter 33. Consequently, only the up-sweep counter 33 operates duringthe increase in frequency of the R. F. carrier wave, and only thedown-sweep counter 32 operates during the decrease in frequency of thecarrier wave. Counters 32, 33 feed into a common load 31 comprised of acapacitor 38 and resistor 39.

The sensitivity of the up-sweep counter 33 is made greater than that ofdown-sweep counter 32 by so choosing condensers 34, 35 that thecapacitance of condenser 34 is greater than that of condenser 35. Aratio of the two of 1.42 is satisfactory so that the distance voltageswill not cancel out in the common load 31.

Gaan'erationV of speed voltage With airplane l moving toward the targetll' at some given speed, then as; previously explained, a Dopplerfrequency willV be obtained which will be proportional to the speed. Thepresence of the Doppler effect will increase the frequency of theresultant beat-note during the down-sweep and decrease it duringtheup-sweep. Thus, since the frequencies applied to counters 32 and 33are no longer'thesame, it. being remembered that the. twocountersroperate alternately, the load circuitY becomes unbalanced and aresultant average current component iiows through resistance. 39; whichis proportional to speed. Since the sweep of counters 32 and 33 isphased with the R. F. sweep of the carrier wave so that during thedown-sweep only the negative counter is operating, then the resultingaverage current component, which is proportional to speed, is negative.

Generation of distance voltage If the distance from the aircraft. l0 tothe target il is constant and condenser 34 is larger than condenser 35,then the average current component through resistor 39 Will beunbalanced in a. positive direction, since the sensitivity of counter33; is greater than counter 32. Since condenser 34 is in the up-sweepcounter circuit, the resultant average current component, which isproportional toA distance, is positive. Hence, for any fixed distancefrom the target I l, a positive average current component will appearthrough resistor 39 whose amplitude is proportional to the slantdistance.

Combination of speed and distance voltages Under a condition of unequalcounter capacitors 34, 35, and in the presence of the Doppler eifect, itis seen that there now appears through resistor 39 a negative averagecurrent component due to speed', and a positive average currentcomponent due to distance. As has been previously stated, the resultantbeat-note during the up-sweep fu is proportional to (D-S), and duringthe down-sweep, fd is proportionalv to (D+S). Since counters 32, 33 havea linear characteristic, the up-sweep voltage Vu developed across thecommon load 31 will be vdw 12)v Similarly, the down-sweep voltage Vddeveloped across this commen load 3T will be impedan@ (is) whereCu=UpswcepV capaci-tor 34 Cz=Downsweep capacitor 35 R=Total loadresistorof counter fu=Upsweep frequency fd1=Dewnsweep frequency ES=Square wavevoltage from limiter 3| The load voltage wi-ll be mun-Oife 14) But aspreveiously developed where DA and S; are slant distance and; speed,respectively, and K and K1. are constants. Substituting =[Km-adm-Klwcasm15) Since K, K1, Cu, Ca, R and Es are all constants, they may be lumpedand the equation becomes Equation 16 indicates that the load 31 has apositive voltage proportional to` slant distance and a negative voltageproportional to slant speed. If the speed is constant,l only thevoltage. due to distance varies appreciably as the plane I0. approachesthe target H.. (Actually, of course, the measured slant speed variesslightly asa func'- tion of cosi-ne 0.) Hence as the distance to thetarget decreases, the total load` voltage across resistor 39 decreases,i.v e., the load voltage becomes less positive. It is to be noted,however, that the voltages from the up-sweep and downf' sweep counters33 and 32.v each contain components of both speed and'y distance; hence,it cannot be said' that the distance or speed comes from either counteralone.

Application of resultant speed and distance voltage Thel resultant loadvoltage appearing across capacitor 38 is applied to the grid of acathodefollower stage t2. The cathode follower tube 42 has unity gainand essentially zero phase shift, so a voltage of the same magnitude andpolarity appears at its cathode as is present at any instant oncapacitor 38. The cathode follower 42* also serves as an impedancetransformer andl as a unidirectional isolation between capacitorA 38 anda relay amplifier tube 43. The cathode of the relay amplifier 43 islconnected to the cathode of the cathode follower 42 so that the cathodeof the relay amplifier 43 is also at the same potential as loadcapacitor 38 at all times.

Correction for distance intercept d-a and range lead: compensation Inorder to compensate for the condition that the target distance at. zerospeed is not` zero, it is necessary `to introduce a correction voltageso as to cause bomb release relay 21 (which is connected` i'n the anodecircuit of the relay amplifier 43') to operate. earlier by an amountequal. to the distance intercept, as previously explained. Since thislatter factor varies. with altitude, the correction voltage applied mustlikewise vary with altitude. This, control is therefore ganged with thesweep width control! in the manually operated altitude compensationswitch 26 and supplies a positive potential increasing with altitudefrom a potentiometer 46 to the grid of relay ampliiier 43.

If it is desired. that. the bomb fall from 0 to feet short of the,target, a. further positive voltage in addition to that derived tocorrect for the distance intercept must be applied to the grid of relayamplifier 43. The amount of the range lead compensation potential:necessary for a given range lead will, of course, also vary with thealtitude of the plane. Hence, such potential can be conveniently taken01T a range lead potentiometer 4.1i which is varied jointly by thealtitude compensation switch 216 and a range lead control' 48.

The positive voltages obtained from potentiometers 46 and 41 aresupplied in an additive -manner to the grid of relay amplier 43, and

the two determine the total xed positive bias on this grid.

Time of operation of bomb release relay The diierential output voltagewhich appears across capacitor 33 is applied to the cathode of the relayamplifier 43. As previously explained, this voltage will have a positivevalue variable with the target range and range rate. Also, the grid ofamplifier 43 is set at a xed positive potential dependent upon altitudeand range lead. Now, as airplane l closes the range to the target Il,the positive voltage appearing at the cathode of relay amplier 43 willgradually decrease with the result that this cathode becomes lesspositive with respect to its grid, ultimately reaching the cut-on pointof the tube 43 and initiating current flow in the anode-cathode circuitof this tube. The winding of the bomb release relay 21 is connected inthis latter circuit and, of course, requires a minimum amount of currentin order to cause the relay to pull in its armature and close thebombrelease circuit and associated bomb release mechanism which itcontrols. Thus, when the cathode of the relay amplifier 43 has becomesufliciently negative with respect to its grid by a reduction in thepositive voltage across the capacitor 38 to cause this minimum currentto flow through the cathodeanode circuit of relay amplifier 43, therelay 21 will be actuated. To put it in another way, since the windingof relay 21 requires a certain minimum amount of current to operate itsarmature, there is one minimum voltage difference between the grid andcathode of the relay tube 43 which will trigger the relay 21. For eachpreset altitude and range lead, therefore, there is only one minimumvoltage from capacitor 38 which will throw relay 21. Hence, Equation 16becomes K3D-K4S=Ke n n D- K3S K3 (17) With K4 E M and K5 afd Equation 17becomes identical to Equation 2, i.e., D=MS+cZo Thus it is seen that theapparatus solves the equation which the basic theory evolved as therequirement for bomb release at the proper instant. f

The apparatus so far described is conventional; hence, no noveltythereto per se is claimed in this application. As previously stated inthe opening of this specication, this invention relates to animprovement in apparatus of the type which has been described, whereinthe pilot of the aircraft is furnished with a warning signal in advanceof actuation of the bomb release relay which thus indicates to him thatthe bomb release apparatus is functioning properly in computing the timeof bomb release, and thus 10 assures him that the bomb load will bereleased automatically at the proper time.

In a preferred embodiment of the invention, the warning signal isactuated by the same output utilized to actuate the bomb release relay,namely, the voltage output from the differentially connected counters 32and 33.

The warning unit comprises an amplifier 5|, relay 52, and alarm device53. The alarm device, indicated only in block form, may be of anysuitable type operated electrically such as, for example, a signallight, bell, vibrator, etc.

The cathode of amplifier 5l is connected by a line 54 to line 55 whichties the cathodes of tubes 42 and 43 together. Hence, the voltageappearing at the cathode of tube 5I will be the same as that appearingat the cathodes of tubes 42 and 43, which voltage, as previouslyexplained, is the same as the voltage across the load capacitor 36.

The coil of relay 52 is connected in the anode circuit of tube 5| andwhen this relay is energized suiiiciently to close the contacts 52a, thecircuit between the alarm device 53 and its source of power supply 56 iscompleted, thus actuating the alarm and giving the pilot the warningsignal referred to above.

A source 51 of positive potential is applied to the grid of tube 5I.This biasing potential is so selected that it will be much higher thanthe total positive bias which is placed upon the grid of tube 43 fromthe potentiometers 46 and 41.

The warning device operates as follows: When the pilot first sights thetarget, and turns on the FM radio ranging apparatus, previouslydescribed, which computes the time for actuation of the bomb releaserelay as a function of target range and range rate, the positive outputappearing on load condenser 38, appearing also at the cathodes of tubes42 and 43 will be comparatively high because at such time, the targetrange is comparatively great. At such time as aforedescribed, thecathode of tube 43 will be exceedingly positive with respect to its gridand, hence, tube 43 will be held in a non-conductive state. The highpositive output appearing at the cathode of tube `5| will also be sopositive with respect to its grid that this tube will like- Wise be in anon-conductive state. However, as soon as the target range begins todecrease with the approach of the plane Il) to the target Il,

the positive potential of the cathode of tube 5I will begin to drop.Thus, since the grid tube 5l is provided with a positive bias muchhigher than that applied to the grid of tube 43, tube 5I will reachconduction much earlier than tube 43. Thus, relay 52 in the anodecircuit of tube 5| will pull in ahead of relay 21, and the alarm device53 will be actuated in advance of ,the bomb release relay 21 which, aspreviously explained,

is not energized until tube 43 is rendered conductive.

In conclusion, it is to be understood that while the embodiment of theinvention which has been described is to be preferred, changes in theconstruction and arrangement of parts may occur to those skilled in theart. The present embodiments are therefore to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The invention described herein maybe manufactured and used by or for theGoverment of the United States of. America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. In an automatic load release system for an aircraft, means fortransmitting a frequency modulated radio Wave from the aircraft towardsa target, means for receiving the target reiiected wave, means formixing the transmitted and reflected waves to produce a beat frequencywave, means for deriving an output signal from said beat frequency waveproportional to the target range and range rate of change with respectto the aircraft, first and second control' tubesy each having input andoutput circuits, a load release device responsive tothe current in theoutput circuit of said first tube for releasing the load from theaircraft, an alarm device responsive to the current in the outputcircuit of said second tube, means for applying said signal to the inputcircuits of said tubes, and means differently biasing said tubes.

2. In an aircraft automatic load release system, means for transmittinga frequency modulated radio Wave from the aircraft towards a target,means for receiving the target reflected wave, means for mixing thetransmitted and reected waves to produce a beat frequency wave, meansfor deriving an output signal from said beat frequency wave variablewith target range and range rate of change with respect to the aircraft,first and second control tubes, a load release device responsive to thecurrent in the cathode-anode circuit of said first tube for re'- leasingthe load from the aircraft, an alarm device responsive to the current inthe cathodeanode circuit of said second tube, means for applying saidsignal to said tubes to control the current in their respectivecathode-anode circuits, and means biasing said tubes differently beyondcut-oif so that said second tube reaches `conduction in advance of saidfirst tube.

3. In an aircraft automatic load release system, means for transmittinga frequency modulated radio Wave from the aircraft moving towards atarget to said target, means for receiving the target reflected Wave,means for mixing the transmitted and reflected waves to produce the beatfrequency wave, means for deriving a voltage from said beat frequencyWave variable with target range and range rate of change with respect tothe aircraft, first and second control tubes, a load release devicecontrolled by the output of said rst tube for releasing the load fromthe aircraft, an alarm device controlled by the output of said secondtube, means applying said variable voltage to the inputs of said tubes,fixed voltage means biasing said second tube beyond out-off, and fixedvoltage means biasing said first tube further beyond cut-off than saidsecond tube so that as said variable voltage changes, said second tubewill reach conduction in advance of said first tube.

4. In an aircraft automatic load release system, means for transmittinga frequency modulated radio wave from the aircraft moving towards atarget to said target, means for receiving the target reected Wave,means for mixing the transmitted and reflected Waves to produce thedifference frequency Wave, means for deriving a control voltage fromsaid difference frequency wave variable with target range and range rateof change with respect to the aircraft, first `and second control tubes,a load release device controlled by the output of said rst tube forreleasing the load from the aircraft, an alarm device controlled by theoutput of said second tube, a biasing voltage for each of said tubes,and circuit means applying said biasing and control voltages to jointlycontrol conduction of said tubes, the first of said tubes being biasedfurther beyond cut-off than said second tube whereby said second tubewill reach conduction in advance of said first tube.

5. In an aircraft automatic load release system, means for transmittinga frequency modulated radio wave from the aircraft to a target, meansfor receiving the target reflected wave, means for mixing thetransmitted and reflected Waves to produce the difference frequencywave, means for deriving an output from said difference frequency Wavevariable with target range and range rate of change with respect to theaircraft, first and second control tubes each having an input and outputcircuit, a relay connected to the ouput circuit of said rst tube forreleasing the load from the aircraft, an alarm relay connected to theoutput circuit of said second tube, an alarm device controlled by saidalarm relay for indicating 'the operation of said releasing relay, meansapplying said variable output to the input circuits of said tubes tocontrol conduction thereof, and means biasing said tubes differentlybeyond cut-off whereby said second tube reaches conduction in advance ofsaid first tube.

6. In an aircraft automatic load release system, means for transmittinga frequency modulated radio wave from the aircraft to a target, meansfor receiving the target reiiected wave, means for mixing thetransmitted and reflected Waves to produce the difference frequencywave, means for deriving a voltage from said difference frequency Wave,said voltage becoming less positive as the target range decreases, firstand second control tubes, a load release device responsive to the outputof said first tube for releasing the load from the aircraft, an alarmdevice responsive to the output of said second tube, means applying saidvoltage to said tubes to control conduction thereof, and means biasingsaid tubes differently beyond cut-off whereby said second tube reachesconduction in advance of said first tube.

'7. In an aircraft automatic load release system including means forproducing an output signal proportional to the range and range rate ofchange between a target and the aircraft, the combination comprisingmeans responsive to the output signal for releasing the load from theaircraft, means responsive to the output signal for indicating theincipient operating condition of said releasing means, means forrendering said indicating means operable in advance of the operation ofsaid releasing means, said last named means including a pair ofdifferently biased electron discharge tubes controlling the operation ofsaid indicating means and said releasing means respectively.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,268,587 Guanella Jan. 6, 1942 2,268,643 Crosby Jan. 6, 19422,301,929 Budenbom Nov. 17, 1942 2,441,657 Blitz May 18, 1948 2,454,009Sanders Nov. 16 1948

